supernova

(SN)

(stellar explosion larger than a nova)

A supernova (SN) is a stellar explosion more energetic than
an ordinary nova. It is often briefly brighter than
an entire galaxy, and fades away over a period of weeks
or months. Energy emitted can be on the order of
the Sun's entire lifetime.
The explosion ejects most of the star's material at
up to 10% of the velocity of light, sweeping up
interstellar gas and dust, which is called
a supernova remnant.

Supernovae are thought to come from a collapse of the
core in large stars (core collapse supernova), or in
a white dwarf accreting enough matter that it collapses
(thermonuclear supernova, Type Ia).

The absolute brightness of a Type Ia supernova is thought
to be determinable from its spectrum and fade time.
These factors plus its apparent brightness
are used to estimate distance to the supernova and
thus to its galaxy.